John A. Lind wrote:
>This was much my thinking . . . SLR's already have retrofocus lenses (does
>that qualify as being "telecentric" ??) for the lenses shorter than could
>be otherwise mounted in front of the reflex mirror. So what's the
>deal? We have a new name for "retrofocus" now? Am I missing something???
>
>
Possibly. Retrofocus simply refers to a design where one of the nodes is
outside the physical lens. It happened first with teles. look at the
eSIF lens chart. Starting with the 135mm lenses, the front node is out
in front of the lens. The 135/3.5 is 73mm long, add 46mm register
distance to get 119mm. The focal length is 16mm longer than the distance
from the front of the lens to the film.
Later, the same trick was done in reverse to put the rear node behind
the rear element for wide angle lenses for SLRs. Notice nothing there
says anything about the angle at which the light from the lens hits the
"sensor". We non optical designers may assume that a lens with its rear
element further from the sensor has light hitting it at a closer to
right angle, but that may not be true.
Telecentric, on the other hand, says nothing about the distance of the
lens elements from the sensor, only that the light hitting the sensor
should be at near right angles. I assume that the combination of
telecentric and retrofocus may be more difficult that either one by itself.
Moose
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